This invention relates to modems for enabling data communication between multiple data signal sources over a combination of analog and digital telephone company lines, and more particularly relates to techniques for processing signals at the inputs and outputs of such modems.
FIG. 1 describes prior art for enabling communication between computers connected to analog telephone lines and computers coupled together over a digital network. Digital computers C1-C12 communicate with a telephone company network TC1 via conventional modems M1-M12, respectively. Each of computers C1-C12 is a separate source of digital data signals representing digital data. In a well known manner, modems M1-M12 convert the digital data signals into corresponding analog telephone signals for transmission over conventional pairs of analog telephone wires A1-A12, respectively. The analog telephone wires typically extend to a telephone company central station at which the analog telephone signals are converted to digital telephone signals for transmission and switching through the telephone company digital network. The telephone company typically uses a digital conversion called CODEC which samples the analog telephone signals at 8,000 samples per second using 8 digital bits per sample. The resulting digital telephone signals typically are transmitted over a four wire digital telephone span line commonly called a T1 line. Each T1 telephone line carries 24 digital channels that are multiplexed onto the T1 lines by a well known time division multiplex technique. For each of the digital channels, the telephone company adds layers of call set-up information according to the conventions established by the International Standards Organization (ISO). The call set-up information typically includes the telephone number being called.
Assuming the twelve digital channels of information representing data from computers C1-C12 are directed to a single user location, they typically will be switched to digital telephone line T1 which is terminated by a PBX box P1 at the user""s location. PBX box P1 demultiplexes the 24 channels of digital telephone signals on line T1 and converts each digital signal to a corresponding analog telephone signal. Thus, the 24 channels of multiplexed digital telephone signals on line T1 are divided into 24 separate analog telephone signals on 24 separate pairs of analog telephone lines. Twelve pairs of the analog telephone lines A13-A24 are represented in FIG. 1 as inputs to conventional modems M13-M24. Modems M13-M24 are identical to modems M1-M12.
For incoming calls on line T1 from computers C1-C12, modems M13-M24 demodulate the analog telephone signals and covert them into digital data signals. The digital data signals typically are in a serial digital form suitable for transmission through an RS-232 digital port. Each of the twelve channels for modems M13-M24 may be connected to a terminal server TS1. Such servers have software and an output port which distribute data on a local area network, such as token ring network TRN1, among computers, such as computers C13-C24.
As shown in FIG. 1, analog telephone signals are used to represent digital data at two different points in the system, i.e., analog conductors A1-A12 and analog conductors A13-A24. Conversion between digital and analog signals occurs twice irrespective of whether a telephone call is incoming or outgoing.
For an incoming call from computers C1-C12, modems M1-M12 convert the digital data signals from the computers to analog telephone signals that are transmitted to the telephone company network TC1. Network TC1 converts the analog telephone signals to corresponding digital telephone signals. At the receiving station, PBX unit P1 converts the digital telephone signals to analog telephone signals that are demodulated by modems M13-M24 to generate network digital data signals suitable for use by server TS1 and computers C13-C24.
For outgoing calls from computers C13-C24, the network digital data signals generated by the computers are converted to corresponding analog telephone signals by modems M13-M24. The analog telephone signals are converted by PBX unit P1 to digital telephone signals suitable for transmission on the T1 line. After transmission in digital form, network TC1 converts the digital telephone signals into analog telephone signals that are transmitted over analog telephone lines A1-A12. The analog telephone signals are demodulated by modems M1-M12 and are converted to digital form for use by computers C1-C12.
The data from computers C1-C24 appears in RS-232 form at two points in the system, i.e., on one set of conductors connected to modems M1-M12 and on another set of conductors connected to modems M13-M24. Before signals originating at computers C1-C12 can be used on network TRN1, the RS-232 form of the signals at modems M13-M24 must be converted to blocks of digital data suitable for transmission on network TRN1.
The applicant has found that the prior art requirement for twice converting signals between digital and analog form and twice converting signals to and from RS-232 form in order to allow digital data sources to communicate via telephone company networks is inefficient and expensive. In addition, the need for separate busses for the distribution of data from modems M13-M24 to terminal server TS1 creates time delays and requires substantial duplication of circuitry.
In Hugh E. White, xe2x80x9cA T1-Based DSP Modem For Interfacing Voice And Packet Networksxe2x80x9d (IEEE 1988), an all digital system converts PCM samples on a T1 trunk to and from data bits on a virtual circuit of an X25 trunk. However, the described structure of the system is insufficient to enable the high speed transfer of data between multiple modems with sufficient flexibility to provide efficient utilization of the modems for different applications.
In Paul Desmond, xe2x80x9cPrimary Access adds PAD To Network Access System,xe2x80x9d (Nework World, p. 17, Mar. 4, 1991), certain functions of a network access system are identified. Output from DSP cards which perform a modem function is said to route to DCP cards over an RS-232 interface. The DCP cards are said to perform a packetizing function. This arrangement also is not sufficiently flexible or fast enough to properly utilize the capabilities of the DSP cards.
In order to overcome the deficiencies of the prior art, a primary object of the invention is to create a bus structure that increases the efficiency of data communication between one group of computers connected to analog telephone lines and a second group of computers connected to a local area network.
Another object of the invention is to demodulate telephone signals to form packets of signals that can be stored and analyzed to facilitate the demodulation of the telephone signals.
Still another object of the invention is to reduce the number of conversions between digital and analog form required for communication by digital data sources via telephone company networks employing both analog and digital telephone lines.
Still another object of the invention is to provide an improved bus for transmitting signals between a single digital telephone line and a plurality of modems.
Yet another object of the invention is to provide a bus of the foregoing type which employs a switch enabling bidirectional transmission of either (1) data from or to a telephone line; or (2) control signals under the control of a processing unit.
Still another object of the invention is to provide an improved bus for transmitting signals between a single local area network and a plurality of modems.
Yet another object of the invention is to provide a modem system in which communication channels are coupled from a telephone line to the modems over a circuit switched time division miltiplex bus and in which data is coupled from the modems to a network over a parallel bus.
The invention is useful in a system comprising a multiplexed digital telephone line carrying a digital first telephone signal resulting from modulation by a first analog modem of a first digital computer signal. The signal represents digital first data from a digital first computer. The telephone line also carries a digital second telephone signal resulting from modulation by a second analog modem of a second digital computer signal. The signal represents digital second data from a digital second computer.
The system also comprises a network for transmitting a digital first network signal comprising blocks of digital time-spaced signals representing digital third data from a digital third computer and for transmitting a digital second network signal comprising blocks of digital time-spaced signals representing digital fourth data from a digital fourth computer. In a system of the foregoing type, the applicants have discovered that improved bilateral transmission of the digital data between the digital telephone line and the network can be achieved by using a unique combination of components, including a unique bus structure. Telephone control means responsive to the first telephone signal are used for generating a digital first telephone bus signal representing the first data. The control means also are responsive to the second telephone signal for generating a digital second telephone bus signal representing said second data.
Network control means responsive to said first network signal are used to generate a digital first network bus signal representing the third data The network control means also are responsive to the second network signal for generating a digital second network bus signal representing the fourth data.
First modem means responsive to the telephone control means and the network control means modulate the first network bus signal to form a digital third telephone bus signal representing the third data. The first modem means also demodulate the first telephone bus signal to form a digital third network bus signal representing the first data.
Second modem means responsive to the telephone control means and the network control means modulate the second network bus signal to form a digital fourth telephone bus signal representing the fourth data The second modem means also demodulate the second telephone bus signal to form a digital fourth network bus signal representing the second data.
Telephone bus means responsive to said telephone control means transmit the first and third telephone bus signals between the telephone control means and the first modem means. The telephone bus means also transmit the second and fourth telephone bus signals between the telephone control means and the second modem means.
Network parallel bus means responsive to said network control means transmit the first and third network bus signals between the network control means and the first modem means. The network parallel bus means also transmit the second and fourth network bus signals between the network control means and the second modem means.
By using the foregoing apparatus, the first and third computers bilaterally communicate while said second and fourth computers bilaterally communicate via the telephone line. The unique bus structure, in combination with the other components, enables computers connected to conventional analog and digital telephone lines to communicate with computers connected by a network with a degree of accuracy and economy unattainable by prior techniques.
According to another aspect of the invention, the third and fourth network bus signals are processed. By using this technique, call setup information included in the telephone signals can be used to select parameter signals representing demodulation standards. The demodulation of the telephone signals is executed according to the selected demodulation standard.
According to another aspect of the invention, apparatus is provided for enabling bilateral transmission of digital data between a digital telephone line carrying multiple data channels and a network. The apparatus includes a plurality of modems, as well as a circuit switched time division multiplex bus and a parallel bus. Telephone control means couple the data channels to the time division multiplex bus. Network control means couple the parallel bus to said network. By using such unique structure, the synchronization of information in the data channels can be maintained by the time division multiplex bus, and the bandwidth available on said parallel bus is available to each of said modems.
By using the fore going techniques, computers linked by telephone lines and networks can communicate with a degree of control and speed unattainable by prior techniques.